A diffraction grating using liquid crystals can be used as an optical device having polarization dependence, which emits light with different optical characteristics in accordance with a polarization state of incident light by using the refractive index anisotropy that the liquid crystals have, i.e. a difference Δn between the refractive index no for ordinary light and the refractive index ne for extraordinary light. For example, as illustrated in FIG. 1, in a liquid crystal element 10 having two parallel transparent substrates 11a and 11b, one transparent substrate 11a has a diffraction grating 14 in which a cross section of one surface of the transparent substrate 11a is in a periodic concavo-convex shape, and is arranged opposite to the other transparent substrate 11b. Further, an alignment layer 12a formed on the surface of the transparent substrate 11a is arranged opposite to an alignment layer 12b formed on the surface of the transparent substrate 12a, and a liquid crystal layer 13 filled with a liquid crystal material is placed in a gap between the two transparent substrates. Polyimide, which is an organic material, is appropriately used as the alignment layer. Particularly, a method of providing an alignment layer by rubbing a surface on which polyimide is formed and adjusting the alignment direction of liquid crystal molecules which are in contact with the surface of the alignment layer has been known. Also, an oblique deposition method that performs deposition of an alignment layer of an inorganic material such as SiO2 on the surface of the opposite substrate in a predetermined slanting direction from the normal line of the substrate surface has been known.
In this case, the liquid crystal layer 13 is made using, for example, a liquid crystal material having positive dielectric anisotropy, the longitudinal direction of liquid crystal molecules of the liquid crystal material is approximately level with the transparent substrate surface and is in a direction that is parallel to the length direction (Y-axis direction) of the concavo-convex of the diffraction grating 14, and the refractive index ns of the transparent substrate 11a made of an isotropic optical material is made approximately consistent with the refractive index no for ordinary light of the liquid crystals. If light that travels in parallel to the Z-axis direction is incident to the liquid crystal element 10, the light polarized in X-axis direction does not realize the difference in refractive index between no and ns and thus is transmitted straight through the liquid crystal element with almost no diffraction, while the light polarized in Y-axis direction realizes a difference in refractive index between ne and ns and thus diffraction phenomenon is revealed. In the liquid crystal element that creates the polarization dependence, the alignment state of the liquid crystals may be changed by arranging a further transparent conductive layer such as ITO or the like between the liquid crystals and applying an AC voltage to the conductive layer. Also, the liquid crystals may be made of a material having the characteristic of positive dielectric anisotropy or a material having the characteristic of negative dielectric anisotropy, and the liquid crystal materials, the alignment methods, and the like, may be combined in accordance with the optical uses. In this case, a concave portion 15 and a convex portion 16 of the diffraction grating (corresponding to concavo-convex portions) 14 indicate a concave portion and a convex portion of the liquid crystal layer 13. Unless specifically explained in the liquid crystal element according to an embodiment of the present invention, the concave portion and the convex portion mean the concave portion and the convex portion of the liquid crystal layer in all.
In the diffraction grating structure, if the length of one period of the concavo-convex portion (hereinafter referred to as “grating pitch”) is decreased (or shortened), the diffraction angle (i.e. the angle between the Z-axis direction and the diffraction direction) of the light, which travels and is incident in the Z-axis direction of the FIG. 1 and is polarized in parallel to the Y-axis direction, is extended. In this case, for example, the light, which is polarized in parallel to the X-axis direction in which no diffraction occurs, is incident to another optical device that is arranged in direct transmission direction at high transmission rate. On the other hand, the light, which is polarized in parallel to the Y-axis direction, is diffracted at high diffraction angle and thus is not to be incident to other optical devices to heighten the extinction ratio of the light that is transmitted in the straight direction. Also, by extending the diffraction angle, for example, the diffracted light becomes stray light which is difficult to be incident to other optical devices arranged in the straight direction, and thus the quality of the optical system is heightened.
Next, it is exemplified that the liquid crystal element 10 is arranged in an optical head device 20 as shown in FIG. 2. The light that is emitted in the direction of an optical disk 25 through direct polarization of the light source 21 in the X-direction is transmitted through the liquid crystal element 10 without diffraction. The light, which becomes a parallel light through a collimator lens 22, is transmitted through a ¼ wavelength plate 23 to be a circularly polarized light that turns to the right, and reaches an information recording surface 25a of the optical disk 25 after being condensed by an object lens 24. The light reflected from the information recording surface 25a is transmitted through the object lens 24 as the circularly polarized light that turns to the left, becomes a linear polarized light in the Y direction by the ¼ wavelength plate 23, and then is diffracted by the liquid crystal element 10 to reach a light receiving device 26. At this time, since the diffraction angle becomes larger as the grating pitch of the liquid crystal element 10 is narrower, the reflected light, if the diffracted light can be deflected to have a large diffraction angle, may not be a stray light with respect to, for example, a semiconductor laser that is the light source 21.
Next, it is considered that the liquid crystal element 10 is designed by specifying the phase difference (i.e. length difference between light paths) of the transmitted light. If the liquid crystal molecules are uniformly aligned within the liquid crystal layer 13, the length difference between the optical paths is realized by adjusting a liquid crystal material and the depth of the diffraction grating. The length difference between the optical paths when the light that travels in the Z-axis direction is transmitted through the concave portion 15 and the convex portion 16 can be explained as follows. It is assumed that the refractive index for ordinary light of the liquid crystal material with respect to the wavelength λ of the incident light is no, the refractive index for extraordinary light is ne, the refractive index of a grating material composed of an isotropic material is ns, and a difference in refractive index between the liquid crystal material and the grating material is Δn0=|n0−ns|, Δne=|ne−ns|. In this case, if the light in an ordinary-light direction of the liquid crystal material, i.e. in a fast axis direction, is incident, the length difference between optical paths that occurs in the diffraction grating having a grating depth d becomes Δno·d, and if the light in extraordinary-light direction of the liquid crystal material, i.e. in slow axis direction, is incident, the length difference between optical paths that occurs in the diffraction grating having a grating depth d becomes Δne·d. For example, on condition that the phase difference is (2m+1)λ/2 (where, m is an integer that is equal to or larger than 0) in order to heighten the diffraction efficiency of ±1-order diffracted light in the tetragonal diffraction grating, it is preferable to adjust the grating depth d by specifying the liquid crystal material. By doing this, the kinds of usable liquid crystal materials can be increased. If the alignment state is not uniform in the liquid crystal layer, the depth of the diffraction grating or the like may be adjusted in accordance with the alignment state to cope with the non-uniform alignment state.
Also, when the light having a wavelength λ is vertically incident to the surface of the liquid crystal element having a diffraction grating of the grating pitch P as relations between the grating pitch of the diffraction grating and the diffraction angle, the diffraction angle θ of the Q-order diffracted light against the normal line of the surface of the liquid crystal element (=light traveling direction) becomes as in Equation (1).sin θ=Qλ/P (where, Q=±1, ±2, . . . )  (1)
From this, for example, if the diffraction angle is enlarged with respect to the light having a short wavelength, for example, such as 400 nm band that is adapted to an optical head device for a high-density DVD, it is necessary to further narrow the grating pitch. Also, the concavo-convex portion of the diffraction grating is not limited to a tetragonal shape, and may be in a blazed grating shape or in a shape in which the blaze is formed to approximate the shape of stairs. In this case, the light quantity of the diffracted light in one direction may be extended to heighten the optical use efficiency.
In the above-described diffraction grating, the diffraction angle can be extended by making the grating pitch, for example, equal to or less than 10 μm (i.e. several μm) to meet the wavelength of the incident light. However, in the case of forming an alignment layer on the diffraction grating by a rubbing method in the related art, the fiber for rubbing is not sufficiently in contact with the groove (that is the portion contacting the convex portion 16 of the liquid crystal layer) of the transparent substrate 11a due to the narrow grating pitch, and thus the anchoring force becomes insufficient. Accordingly, the liquid crystal molecules on the surface of the convex portion 16 of the liquid crystal layer are not sufficiently aligned to cause the occurrence of alignment non-uniformity. Also, the fiber of a rubbing cloth is generally in the range of several tens of μm, and thus it is difficult to control the alignment of the liquid crystal molecules by rubbing the diffraction grating structure having the grating pitch in the range of several tens of μm, and particularly, equal to or less than 20 μm. Due to this, there is a problem that the optical characteristics such as diffraction efficiency of the light incident to the liquid crystal element, the polarization state of the diffracted light, and the like, cannot be stably obtained.
In order to reduce the influence of the alignment non-uniformity as described above, Patent Document 1 discloses a method for forming an alignment layer on a transparent conductive layer and forming a diffraction grating pattern thereon by an electron-beam resist. In this method, the alignment layer is exposed from a portion from which the resist is removed, and liquid crystal molecules which are in contact with the alignment layer are vertically aligned.
Also, as a method for obtaining a grating pitch of several μm without rubbing, a method of aligning liquid crystals using an imprint method which is a forming process technology which transfers the concavo-convex pattern of a grating pitch of several μm onto a resin layer by pressing a stamper, in addition to a method of straightly processing a substrate surface by photolithography and etching. The imprint method may be a thermal curing type thermal imprint method or an optical imprint method for curing a resist material by illuminating ultraviolet rays. In Patent Document 2, a liquid crystal element and a method of manufacturing the liquid crystal element, which forms a transparent conductive layer on the surface of a diffraction grating of a grating pitch of several μm, interposes liquid crystals between flat substrates having transparent conductive layers opposite to each other, and aligns the liquid crystal molecules by applying a voltage thereto to form polymer liquid crystals, have been reported. In this method, it is not necessary to form an alignment layer, and thus the alignment non-uniformity due to rubbing does not occur.    Patent Document 1: JP-B-6-052348    Patent Document 2: JP-A-2005-353207